The present invention is directed to aqueous ink-jet ink compositions exhibiting rapidly reduced pH as well as increased viscosity and precipitation of an ink colorant following exposure of the ink composition to a radiation source.
Computer printer technology has evolved to a point where very high-resolution images can be transferred to various types of media, including paper. One particular type of printing (referred to generally as ink-jet printing) involves the placement of small drops of fluid ink onto a media surface in response to a digital signal. Typically, the fluid ink is placed or jetted onto the surface without physical contact between the printing device and the surface. Low cost and high quality of the output have made ink-jet printing a popular alternative to other computer related forms of printing.
However, even though great improvement in ink-jet printing has been made, along with this improvement, demand for even higher quality ink-jet printing systems has arisen. These needs include higher speed, higher resolution, full color image formation, reduced color-to-color bleed, more precise dot placement, etc. As such, there are several characteristics to consider when evaluating a printer ink. Such characteristics include its ability to print with good edge acuity and optical density, dry time of the ink on the substrate, adhesion of the ink to the substrate, lack of deviation of the ink droplets in flight, resistance of the ink after drying to water and other solvents, long-term storage stability, and long-term reliability without corrosion to the nozzle or clogging. Though the above list of characteristics provides a worthy goal to seek or strive after, difficulties arise in attempting to satisfy all of the above characteristics. Often, a formulation of an ink-jet ink represents a balancing act between improving on one of the above characteristics at the cost of another. Thus, most commercial inks for use in ink-jet printing represent a series of compromises, made in an attempt to achieve at least an adequate response in meeting all of the above listed ink requirements. Additionally, and of particular importance to the present invention, the optimal rheological characteristics of ink-jet inks (and most forms of ink generally) in their latent forms differ from the optimal rheological characteristics of ink-jet inks in their applied forms. This is because ink-jet inks in their latent form must perform well under the extreme physical stresses imposed during the jetting process, while ink-jet inks in their applied form must adhere quickly and remain static. Therefore, ink-jet ink compositions and application methods that are capable of achieving desired rheology in both the latent and applied form are currently the focus of much research.
Similar needs to improve latent and applied ink rheology and generally increase production rates and print quality in the printing industry have lead to a recent surge in the use of radiation initiated photo-polymerization in a variety of printing processes. These processes typically use ink compositions containing a pre-polymer such as unsaturated alkyl chains, epoxy resins, epoxy monomers, or the like, and a catalyst in the form of a diarylsulfonium or triarylsulfonium salt, which can initiate cationic polymerization of the pre-polymer upon exposure to radiation. The cationic polymerization of the pre-polymers contained in these inks can be initiated by an irreversible photo-fragmentation that occurs in the diarylsulfonium and triarylsulfonium salt compounds upon exposure to radiation. This photo-fragmentation process causes the onium salt photo-initiated catalysts to produce, in situ, a BrØnsted Acid, capable of initiating cationic polymerization. The resultant polymerization of the pre-polymers contained in these inks substantially improves print quality and applied ink rheology by increasing ink viscosity (a quality of fluid that prevents the fluid from flowing when subjected to a force), which in turn increases adherence to the substrate along with smear and water fastness.
However, until the present invention, the use of a photo-initiated acid generator, such as an onium salt, has been impractical for use with ink-jet inks, as the inclusion of pre-polymers typically increase the latent viscosity of an ink-jet ink composition such that it becomes substantially difficult to jet from an ink-jet pen.
It has been recognized that it would be advantageous to develop an ink-jettable ink composition containing a photo-initiated acid generator such that the ink compositions exhibits effective latent rheology and improved applied rheological ink characteristics. This has been accomplished in part because of the absence of pre-polymers used in the prior art, as the photo-initiated compositions used with the present invention provide a different function. Particularly, the ink-jet inks of the present invention can have low initial viscosity such that the ink is readily ink-jettable, and upon exposure to radiation, a rapid drop in pH occurs. This change in pH can rapidly increase the viscosity and ink adherence upon the substrate, which in turn, can improve print quality. Thus, the invention provides compositions, methods, and systems for printing images exhibiting improved print quality, by exposing the above-mentioned ink to a radiation source subsequent to deposition upon the substrate.
Specifically, an ink-jettable ink composition can comprise an effective amount of an ink vehicle; an effective amount of an ink colorant having solubility properties that are pH dependent; and an effective amount of a photo-initiated acid generator, wherein the composition exhibits a decrease in pH upon exposure to an effective amount of radiation, followed by precipitation of the ink colorant.
Further, a method of printing an image on a substrate with increased print quality, water and smear fastness, and optical density can comprise the steps of formulating an ink-jettable ink composition comprising an effective amount of an ink vehicle, an effective amount of an ink colorant having solubility properties that are pH dependent, and an effective amount of a photo-initiated acid generator, wherein the composition exhibits a decrease in pH upon exposure to an effective amount of radiation, followed by precipitation of the ink colorant; jetting the ink composition from an ink-jet pen onto a substrate; and exposing the ink-jetted ink on the substrate to radiation wherein the ink-jet ink composition exhibits reduced pH followed by precipitation of the colorant.
Additionally, a system for printing an image on a substrate with increased ink efficiency can comprise an ink-jet ink pen containing an ink-jettable ink composition comprising an effective amount of an ink vehicle, an effective amount of an ink colorant having solubility properties that are pH dependent, and an effective amount of a photo-initiated acid generator, wherein the composition exhibits a decrease in pH upon exposure to radiation, followed by precipitation of the ink colorant; a substrate configured for accepting a printed image from the ink-jet pen; and an energy source configured for irradiating the printed image such that a decrease in pH is realized resulting in precipitation of the ink colorant.